1. Field of the Invention
The present invention relates to a light intensity ratio adjustment filter placed between a sample and a reference surface of a Fizeau interferometer, an interferometer employing this filter, and a method of measuring light interference, and more particularly to a light intensity ratio adjustment filter employed in an interferometer used when exchanging and measuring samples for which optical reflectance differs greatly, for example, objects of high reflectance such as metal surfaces, and low reflectance objects of such as glass surfaces.
2. Description of the Prior Art
With interferometers, it is generally known that satisfactory high contrast interference wave information is obtained with a small difference in optical intensity when sample light from the sample and reference light are multiplexed, and that the range of difference of, for example, the ratio of optical intensity of the sample light and the reference light is between approximately 5 and ⅕.
Therefore, in a situation in which low reflectance samples must be exchanged with high reflectance samples and measured sequentially, a method is employed in which the reflectance of the reference surface is set to approximately the same as the reflectance of the low reflectance sample, and when measuring the high reflectance sample, an optical attenuation filter is inserted between the reference surface and the sample surface, to attenuate the sample light.
However, with this method, while there is no problem when measuring flat sample surfaces, problems arise in that, when measuring spherical sample surfaces it is difficult to secure space in which to insert the optical attenuation filter between the reference surface and the sample surface, and to ensure that light reflected from the surface of the optical attenuation filter does not negatively effect measurement, the optical attenuation filter is inserted in an inclined manner, and therefore measurement accuracy is lost due to aberration occurring in the sample light (see U.S. Pat. No. 4,820,049 (hereafter referred to as ‘Document 1’)).
Therefore, a method is also known in which the base material of the optical attenuation filter is an extremely thin synthetic resin film (pellicle) on which the optical attenuation coating is applied. Thus, the restrictions on space for insertion of the optical attenuation filter are alleviated, and aberration is of an extent which can be ignored. However, an optical attenuation filter manufactured in this manner is brittle and delicate, and not readily inserted and removed from the light path (see Document 1).
Furthermore, a further method is also known in which the reference surface is coated with a light intensity ratio adjustment film, and the light intensity ratio of the reference light reflected from the reference surface and the sample light passing through the light intensity ratio adjustment film, reflected by the sample surface, and passed again through the light intensity ratio adjustment film, is approximately 1.
However, while the problems related to the optical attenuation filter are solved with this method, there is a new problem in that a plurality of very expensive reference surfaces must be prepared to suit the reflectance of the sample surface, and measurement criteria differ with samples of differing reflectance during measurement, and it is difficult to treat correlation of the measurement results (see Document 1).
To solve this problem, it is possible to achieve a range of light intensity ratio adjustment in which satisfactory high contrast interference wave information can be obtained for the light intensity ratio of sample light and reference light with measurement of both low reflectance samples and high reflectance samples by coating the reference surface with a light intensity ratio adjustment film having a reflectance intermediate between the low reflectance sample and the high reflectance sample (intermediate reflectance).
However, with this method, there is a problem in that, when the light returned from the sample passes through the reference surface, since the reflectance of the reference surface is intermediate reflectance, the intensity of the light reflected again in the direction of the sample increases, reflected interference occurs repeatedly between the sample surface and the reference surface (multiple interference), and phase analysis of the interference fringe obtained is difficult.
An experiment has therefore been conducted in which the structure of the light intensity ratio adjustment film of the reference surface having the intermediate reflectance is modified to ensure that multiple interference does not occur between the sample surface and the reference surface.
For example, a Fizeau interferometer in which the reference surface is coated with a light intensity ratio adjustment film of intermediate reflectance is disclosed in J. Sci. Instrum., 1967, Vol. 44, pp. 899-902 (Surface-coated reference flats for testing fully aluminized surfaces by means of the Fizeau Interferometer)(hereafter referred to as ‘Document 2’), SPIE, 1999, Vol. 3738, pp. 128-135 (Coating requirements for the reference flat of a Fizeau interferometer used for measuring from uncoated to highly reflecting surfaces)(hereafter referred to as ‘Document 3’), and Document 1. These light intensity ratio adjustment films are comprised of, for example, one optical reflection-absorption layer, and one or two dielectric anti-reflection layers, and increase the reflectance of the incident measurement light at the reference surface, and absorb and attenuate light returned from the sample and passing through the reference surface due to the optical reflection-absorption effect of the optical reflection-absorption layer and the anti-reflection effect of the dielectric anti-reflection layers. On the other hand, the light intensity ratio adjustment film comprises a film preventing reflection to ensure that the incident light returned from the sample is not reflected in the direction of the sample and causing multiple interference.
Thus, a reduction in the accuracy of phase information analysis due to light reflected by the reference surface and returned again to the sample surface may be prevented, and the light intensity ratio of sample light and reference light at the reference surface may be readily set to be within the range (for example, between approximately 5 and ⅕) centered on 1, irrespective of the reflectance ratio of the sample.
The optical reflection-absorption layer is comprised of metal layers such as nickel-chrome or bismuth and the like, and the dielectric anti-reflection layers are comprised of metal oxide layers such as titanium oxide or bismuth oxide and the like. One aspect of such a film configuration is known as ‘C&D Coat’.
Since high contrast interference wave information unaffected by multiple interference can be obtained for a variety of samples of various differing optical reflectance using a single light intensity ratio adjustment film with the technology disclosed in Document 1, Document 2, and Document 3, this technology is effective in increasing the efficiency of measurement and the accuracy of measurement.
However, with the technology disclosed in these documents, since the reference surface is coated with the light intensity ratio adjustment film, this film cannot be withdrawn from the light path when it is desired to eliminate the effects due to this film. In other words, when the reference surface and sample surface are both low reflectance glass surfaces, satisfactory interference wave information of good contrast may be obtained without use of the light intensity ratio adjustment film, and conversely, wavefront aberration of the light intensity ratio adjustment film is superimposed on the interference wave information due to a non-uniform distribution of transmissivity in the light intensity ratio adjustment film. With this conventional technology, therefore, it can be said that the interference wave information of low reflectance samples is sacrificed.
Furthermore, with this conventional technology, since the reference surface is coated directly with the light intensity ratio adjustment film, there is the concern that the surface accuracy of the reference surface may be reduced due to the effects of heat applied during application of the coating, and by the effects of membrane stress of the attached light intensity ratio adjustment film, and the like.
Furthermore, with the interferometer configured to allow measurement of sample surfaces with light of differing wavelengths, since the reflection, transmission, and absorption characteristics of the light intensity ratio adjustment film are wavelength-dependent, depending upon the selected wavelength, it becomes difficult to contain the ratio of optical intensity of the sample light and the reference light within a range centered on 1 (for example, between approximately 5 and ⅕), and to control multiple reflection between the reference surface and the sample surface. Thus, there is the concern that a plurality of types of expensive reference surfaces must be exchanged in use.